Radio-frequency identification (RFID) in its various modes is a widely used technology. RFID tags are often needed to be produced at very low cost and ideally consume minimal power. This can result in a restricted reading range. Currently, RFID systems operate mainly at frequencies below 2.4 GHz. The relatively long wavelength means that practical antenna structures are electrically small and no significant directivity can be achieved. The systems either read all tags within a certain range or some kind of addressing scheme is used.
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